Soil organisms drive major ecosystem functions by mineralising carbon and releasing nutrients during decomposition processes, which supports plant growth, aboveground biodiversity and, ultimately, human nutrition. Soil ecologists often operate with functional groups to infer the effects of individual taxa on ecosystem functions and services. Simultaneous assessment of the functional roles of multiple taxa is possible using food‐web reconstructions, but our knowledge of the feeding habits of many taxa is insufficient and often based on limited evidence. Over the last two decades, molecular, biochemical and isotopic tools have improved our understanding of the feeding habits of various soil organisms, yet this knowledge is still to be synthesised into a common functional framework. Here, we provide a comprehensive review of the feeding habits of consumers in soil, including protists, micro‐, meso‐ and macrofauna (invertebrates), and soil‐associated vertebrates. We have integrated existing functional group classifications with findings gained with novel methods and compiled an overarching classification across taxa focusing on key universal traits such as food resource preferences, body masses, microhabitat specialisation, protection and hunting mechanisms. Our summary highlights various strands of evidence that many functional groups commonly used in soil ecology and food‐web models are feeding on multiple types of food resources. In many cases, omnivory is observed down to the species level of taxonomic resolution, challenging realism of traditional soil food‐web models based on distinct resource‐based energy channels. Novel methods, such as stable isotope, fatty acid and DNA gut content analyses, have revealed previously hidden facets of trophic relationships of soil consumers, such as food assimilation, multichannel feeding across trophic levels, hidden trophic niche differentiation and the importance of alternative food/prey, as well as energy transfers across ecosystem compartments. Wider adoption of such tools and the development of open interoperable platforms that assemble morphological, ecological and trophic data as traits of soil taxa will enable the refinement and expansion of the multifunctional classification of consumers in soil. The compiled multifunctional classification of soil‐associated consumers will serve as a reference for ecologists working with biodiversity changes and biodiversity–ecosystem functioning relationships, making soil food‐web research more accessible and reproducible.

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We analyzed the dynamics of the short‐term incorporation of recently fixed carbon into the below‐ground food web in a boreal forest. Five young spruce trees (Picea abies) were pulse‐labeled with 13CO2 and the isotopic label was traced in soil invertebrates during 5 weeks. The freshly fixed plant carbon quickly entered both litter‐located and soil‐located compartments of the detrital food web. Among invertebrates inhabiting the mineral soil layers, a trophic link to the root‐derived C was most pronounced in species with higher δ15N values, suggesting this energy source to be more important in deeper mineral soil horizons. The label appeared faster in saprophagous animals than in predators (the median time lag after labeling was 6 and 12 days, respectively), but the difference was not significant. The label was recovered in 15 of 38 species of saprophagous animals and in 20 of 63 species of predators. Among saprophages, the frequency and intensity of the label was relatively high in endogeic collembolans and in bibionid larvae, but earthworms and enchytraeids were not labeled. Several groups of predators, lithobiid centipedes in particular, quickly acquired the root‐derived carbon, possibly indicating the feeding on live roots or mycorrhizal mycelium. In total, only 35 % of species or genera examined acquired the label. This suggests that majority of invertebrate taxa in the decomposer food web are unlikely to depend heavily on freshly fixed plant carbon provided by roots and root‐associated microorganisms.

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Stable isotope analysis, which is widely used for studying trophic relationships of invertebrates, requires a correct estimation of trophic fractionation factors as well as the proper choice of reference tissues. Different organs and body parts of large insects may vary in the isotopic composition of carbon (δ 13 C) and nitrogen (δ 15 N), though detailed information is available for a limited set of species only. In the field-collected larvae and adults of Oryctes nasicornis (Scarabaeidae) and Uloma rufa (Tenebrionidae), the range of δ 13 C and δ 15 N values for different body parts and organs (muscles, legs, wings, elytra, fat body, gonads, etc.) was 4.8‰ and 2.4‰, respectively. We suggest that the muscle tissue or legs of large insects should be preferably selected for isotope analysis. The isotopic composition of nitrogen in beetles did not depend on the ontogenetic stage and was not affected by metamorphosis. In larvae and adults of Tribolium confusum (Tenebrionidae) reared in the laboratory culture, the trophic fractionation factors of carbon (Δ 13 C) and nitrogen (Δ 15 N) were within the expected values (from −1 to 1.5‰ and from 3 to 5‰, respectively), though depended on the diet. Changes in the δ 13 C values during ontogenesis reflected changes in the lipid content and were strongly correlated with the C/N ratio.

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